B. thuringiensis ("Bt") is a gram-positive bacterium which produces endogenous crystals upon sporulation. The crystals are composed of proteins which are specifically toxic against insect larvae. Three different Bt pathotypes have been-described: pathotype A that is active against Lepidoptera, e.g., caterpillars; pathotype B that is active against certain Diptera, e.g., mosquitos and black flies; and pathotype C that is active against Coleoptera, e.g., beetles (Ellar st al, 1986).
A Bt strain, whose crystals are toxic to Coleoptera, has been described as Bt tenebrionis (U.S. Pat. No. 4,766,203; European patent publication ("EP") 149,162), as Bt M-7 or Bt San Diego (EP 213,818; U.S. Pat. No. 4,771,131) and as BtS1 (European patent application ("EPA") 88402115.5). Two other strains toxic to Coleoptera, BtPGSI208 and BtPGSI245, have also been described (PCT publication WO 90/09445).
The fact that conventional submerged fermentation techniques can be used to produce Bt spores on a large scale makes Bt bacteria commercially attractive as a source of insecticidal compositions.
Gene fragments from some Bt strains, encoding insecticidal proteins, have heretofore been identified and integrated into plant genomes in order to render the plants insect-resistant. However, obtaining expression of such Bt gene fragments in plants is not a straightforward process. To achieve optimal expression of an insecticidal protein in plant cells, it has been found necessary to engineer each Bt gene fragment in a specific way so that it encodes a water-soluble part of a Bt protoxin that retains substantial toxicity against its target insects (EPA 86300291.1 and EPA 88402115.5; patent application Ser. No. 821,582, filed Jan. 22, 1986).